“The combined impact of this back-to-back bleaching stretches for 900 miles, leaving only the southern third unscathed,” said one of the two observers in both years, Professor Terry Hughes, Director of the ARC Centre of Excellence for Coral Reef Studies in Townsville.

“The bleaching is caused by record-breaking temperatures driven by global warming," said Prof Hughes. “This year, we're seeing mass bleaching even without the assistance of El Niño conditions.”

The latest surveys covered more than 5000 miles, and estimations of damage sustained by nearly 800 individual coral reefs closely matched the 2016 results.

“Bleached corals are not necessarily dead corals, but in the severe central region we anticipate high levels of coral loss,” said Prof Hughes’ fellow-observer Dr James Kerry. “It takes at least a decade for a full recovery of even the fastest-growing corals, so mass bleaching events 12 months apart offers zero prospect of recovery for reefs that were damaged in 2016.”

Tropical Cyclone Debbie added to the devastation when in late March it carved a swathe up to 60 miles wide through what had until then been relatively healthy reef.

The GBR reef was “struggling with multiple impacts” the most pressing of which was global-warming, said Prof Hughes.

“As temperatures continue to rise the corals will experience more and more of these events: 1°C of warming so far has already caused four events in the past 19 years." The other two events were in 1998 and 2002.

“Ultimately, we need to cut carbon emissions, and the window to do so is rapidly closing," said Prof Hughes.

The aerial surveys were, as in 2016, backed up by extensive in-water research that has been published in Nature.

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